Lewistown, Montana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

718
FXUS65 KTFX 250551
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1151 PM MDT Thu Apr 24 2025

Aviation Section Updated.

.KEY MESSAGES...

 - Increasing opportunity for afternoon showers and thunderstorms
   across Central and Southwest Montana heading into the weekend.

 - Impactful rain and snow Sunday into Monday across portions of
   Central and Southwest Montana.

 - Another opportunity for precipitation Tuesday into Wednesday
   of next week.

&&

.UPDATE...
/Issued 845 PM MDT Thu Apr 24 2025/

Showers that primarily moved across areas near the ID border late
this afternoon are ending as the associated upper level
disturbance moves east into S-central MT this evening. Skies are
clearing across the area and fog has already developed in the West
Yellowstone area and is likely to remain in the vicinity through
much of the overnight period. East winds remain breezy across the
area this evening and are enhanced across portions of southwest
MT, including over the Bridgers and up the Jefferson and Madison
river valleys, where 25-30 mph gusts are possible through the
early overnight period. Hoenisch

&&

.DISCUSSION...
/Issued 845 PM MDT Thu Apr 24 2025/

 - Meteorological Overview:

Upper level ridging is in place across the region, extending well
northward into Canada, between a high amplitude trough off the
Pacific coast and weaker troughing toward the Great Lakes. A weak
system undercutting the ridge today will keep an opportunity for
showers and perhaps a thunderstorm or two around across Central and
Southwest Montana, but mainly near the Idaho border. The Pacific
troughing nudges eastward and develops into an upper level low
tomorrow off the CA coast, introducing a southerly flow aloft. The
upper low slowly shifts northeastward this weekend into early next
week, bringing several impacts to the region.

The initial impact will be for increasing opportunities for showers
and thunderstorms, first across Southwest Montana Friday afternoon
and early evening, spreading northward into Central Montana as well
for Saturday and Sunday afternoon and evening. Thereafter, as the
core of the upper low passes by Sunday night into Monday, a more
stratiform type of precipitation develops across the region.
Guidance continues to focus on areas near and south of I-90 and east
of I-15 for the most impactful precipitation. Snow levels will fall
through the night from Sunday night into Monday morning as the core
of the upper low approaches and eventually passes. How low snow
levels eventually fall will likely be dictated by precipitation
rate and timing of heaviest precipitation, which is not clear at
this time. Nonetheless, those in the aforementioned area should be
prepared for a period of rain or snow Sunday into Monday, with
potential for snow at pass level.

A third concern over this timeframe will be for increased runoff in
areas where rain overlaps with a snowpack that will be ready to
melt. Warm temperatures during the day this weekend will lead to
snowpack temperatures rising to the freezing mark outside of the
higher elevations. A complicating factor will be for overnight
temperatures getting down toward or below freezing in the mountains.
Confidence in the situation will change as higher resolution
guidance comes in, but the main takeaway at this point is to be
prepared for at least muddy conditions in areas adjacent to snowpack
outside of high elevations.

The upper level low weakens into an open wave Monday and quickly
exits eastward through the day. Upper level ridging looks to attempt
to build in Tuesday and Wednesday, though deterministic guidance
does feature a wave passing through the northwesterly flow aloft
over this timeframe. This will keep at least low-end chance for
precipitation in the forecast over this timeframe. Heading toward
Thursday and Friday, cluster guidance gives good confidence for a
period of upper level ridging, with only uncertainty related to how
quickly the ridging ends up being broken down/ushered east toward
next weekend. -AM

 - Forecast Confidence & Scenarios:

The main point of uncertainty stems from precipitation amounts and
snow levels Sunday night into Monday morning. Ensembles are in
relatively good agreement that the area for highest precipitation
amounts be southeast/south of a Dillon to Lewistown line, with
greatest amounts in the Gallatins, Madisons and vicinity. Most
terrain east of I-15 and south of the MT-200 corridor has a greater
than 50% chance for a half an inch of precipitation from Sunday into
Monday. As mentioned above, where snow levels ultimately fall across
the region will depend on precipitation timing and intensity.
Probabilistic guidance does not support much of a chance for snow at
pass level across Southwest Montana where precipitation looks to
be heaviest, but with deterministic BUFKIT soundings at the
Bozeman Airport suggesting snow levels falling at least briefly to
1,000-1,500 ft AGL (5,500-6,000 ft MSL), it does not seem
unreasonable to think snow at passes in the area could occur.
Given the time horizon of this event still being several days out,
I did not do too much forcing of the forecast to reflect this
opportunity for snow at pass levels across SW MT and vicinity. As
we get closer to the event and confidence in the when/where of
higher precipitation amounts grow, these changes will become more
and more likely. -AM

&&

.AVIATION...
25/06Z TAF Period

A high pressure ridge aloft will continue VFR conditions with
mostly clear to partly cloudy skies through at least 26/06Z for
much of North Central, Central, and Southwest Montana.

However, dense fog and low clouds in the KWYS valley area will
likely continue terrain-obscuring LIFR/VLIFR conditions there
through around 16Z. A disturbance with weak instability will then
bring increasing VFR cloudiness to Southwest Montana after 18Z,
possibly with a few showers and maybe a thunderstorm to the KWYS
area. Thunderstorm coverage should be isolated at most, so have
not included mention of thunder in the KWYS TAF.

A mid-level southeasterly wind flow will increase through the
period, potentially bringing periods of low-level wind shear
through around 15Z, after which the gusty southeast winds should
mix down to the surface. -Coulston

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  30  64  34  70 /   0   0   0  30
CTB  29  61  32  67 /   0   0   0  10
HLN  33  65  37  70 /  10  10  10  40
BZN  29  59  35  69 /  10  10  10  20
WYS  21  58  29  61 /  50  10  30  10
DLN  27  61  38  66 /  10  10  20  20
HVR  30  64  33  71 /   0   0   0   0
LWT  29  54  33  68 /   0  10   0  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$
http://www.weather.gov/greatfalls